[BULK] - [TowerTalk] MOV's

[Steve Katz]

There is a way to measure MOV degradation, for sure, but the device must be
off-line.

MOVs used doped grains of zinc (or other) oxide in series to build up
breakdown voltage (since a single doped grain has a BV of only about 2.2V),
and as surges are applied, the grains not only avalanche as they should, but
begin to break down permanently as conductors.  Obviously, each grain that
fails (and the failure mode is low-resistance, initially) reduces the
breakdown voltage of the string by about 2.2V, and this is a measurable
parameter, using a curve tracer or any number of instruments.  

If you measure the leakage current of a typical 120Vac line MOV device (or
MOV-protected device), it will be very low initially, in the microamps.  As
grains break down permanently, leakage current will continue to increase,
because the remaining grains are approaching, or into, avalanche and
conducting more current.  Just before the device fails altogether, leakage
current can be very high, like 10mA or so, so now the MOV is dissipating
serious power even without any transient (surge) events.

Then, when it gets so hot that the lead bonds melt, the device will "open
circuit," and revert back to no leakage current -- and no protection -- at
all.  However, in most situations, prior to this catastrophic failure, the
leakage current becomes so high that a fuse or circuit breaker will pop,
indicating a problem!  At least, it theory that's what's "supposed" to
happen.  In reality, it often doesn't.

Since testing MOVs is a chore and a nuisance, I simply avoid using any, and
only buy the "unprotected" outlet strips!  

-WB2WIK/6

"Success is the ability to go from failure to failure with no loss of
enthusiasm." -Winston Churchill

> -----Original Message-----
> From:	Jim Jarvis [SMTP:jimjarvis at comcast.net]
> Sent:	Wednesday, August 27, 2003 9:53 AM
> To:	Towertalk
> Subject:	[BULK] - [TowerTalk] MOV's
> 
> 
> 
> >looks like every other decent  MOV protection device... but buried in
> some
> >sand-like material to squelch the flames , when the MOVs go byebye....>
> 
> >we all need to remember that MOVs wear out..with surges..and when do we
> >know they are no longer providing protection ?????
> 
> In deep, dark history, I did some characterization work with MOV's.  They
> tend to fail with accumulated energy.  Polarity matters.   The ones I
> played
> with could take 4 or 5 shots in one direction....and 4 or 5 with the other
> polarity....but 6 or more in any one direction, and it was all over.
> Their damage threshholds progressively diminished with energy, as well.
> That was then.  This is now.
> 
> What's the state of the art?
> 
> Far as I know, there's no way to tell if your
> MOV's have life left in 'em...or when they're dead, for that matter.
> 
> Jim/N2EA
> 
> 
> 
> 
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